1. Field of the Invention
The present invention relates to a valve seal mechanism for providing a seal between two ports of a fluid pressure device when fluid communication between the ports is cut off.
2. Description of the Related Art
Heretofore, valve seal mechanisms are used in fluid pressure devices such as pressure-reducing valves or the like.
FIG. 7 of the accompanying drawings shows a pressure-reducing valve 10 which incorporates a conventional valve seal mechanism. The pressure-reducing valve 10 comprises a valve housing 18 having primary and secondary ports 12, 14 defined therein, a bonnet 22 mounted on an upper end of the valve housing 18, and a handle 24 rotatably supported on an upper end of the bonnet 22.
A valve guide 20 is fitted in a lower end of the valve housing 18 and has a recess 26 in which a valve body 28 is slidably inserted. The valve body 28 is resiliently supported on the valve guide 20 by a helical spring 30. A ring-shaped seal pad 32 of a resilient material is disposed on an upper end of the valve body 28. The valve body 28 supports thereon a rod-shaped stem 34 fixed centrally to the upper end thereof. The stem 34 has a tapered upper end.
The valve housing 18 has a chamber 36 defined therein in communication with the secondary port 14, and the stem 34 extends through the chamber 36. The chamber 36 is defined by a substantially cylindrical wall whose lower end has a seat 38 engageable with the seal pad 32. As shown in FIG. 8 of the accompanying drawings, the seat 38 has an outer wall surface tapered radially inwardly in the direction indicated by the arrow A and an inner wall surface, which defines the chamber 36, tapered radially outwardly in the direction indicated by the arrow A. Therefore, the seat 38 is progressively thinner toward its lower tip end.
As shown in FIG. 7, a diaphragm 40 is clamped between the valve housing 18 and the bonnet 22, and the diaphragm 40 and a recess defined in the upper end of the valve housing 18 jointly define a diaphragm chamber 42 which communicates with the secondary port 14 through a passage 43. A diaphragm holder 44 is mounted centrally on the diaphragm 40 and has a central through hole 46 defined therein. The central through hole 46 includes a downwardly spreading tapered hole 48 which is closed by the tapered upper end of the stem 34.
A spring seat 45 is mounted on an upper surface of the diaphragm 40 around the diaphragm holder 44. A pressure regulating spring 50 has a lower end seated on the spring seat 45 and an upper end seated on a nut 52 threaded over a screw 54 fixed to the handle 24. When the handle 24 is turned, the nut 52 is axially displaced in the direction indicated by the arrow A or B.
When the pressure of a fluid in the secondary port 14 of the pressure-reducing valve 10 is kept at a preset pressure P.sub.2, the tip end of the seat 38 enters and elastically deforms the seal pad 32, providing a seal between the seat 38 and the seal pad 32 (see FIG. 8). Therefore, the primary and secondary ports 12, 14 are disconnected from each other. It is assumed that the tip end of the seat 38 enters the seal pad 32 by a distance L.
When the fluid pressure in the secondary port 14 drops from the pressure P.sub.2 by .DELTA.P, the pressure in the diaphragm chamber 42 drops, displacing the diaphragm 40 in the direction indicated by the arrow A shown in FIG. 9 of the accompanying drawings. The stem 34 and hence the valve body 28 are displaced a distance .DELTA.L in the direction indicated by the arrow A against the bias of the helical spring 30, whereupon the primary port 12 communicates with the chamber 36, allowing the fluid to flow from the primary port 12 to the secondary port 14.
When the fluid pressure in the secondary port 14 increases to the pressure P.sub.2, the pressure in the diaphragm chamber 42 builds up, displacing the diaphragm 40 in the direction indicated by the arrow B. The seat 45 rises, and the stem 34 is displaced in the direction indicated by the arrow B. As a result, as shown in FIG. 8, the seal pad 32 is held against the seat 38, which enters and elastically deforms the seal pad 32, thereby disconnecting the primary and secondary ports 12, 14 from each other.
The distance L by which the seat 38 enters the seal pad 32 varies depending on various elements including the hardness of the seal pad 32, the load on the helical spring 30 (the force applied to the helical spring 30 when the seal pad 32 is held against the seat 38), and the finished configuration of the seat 38. If the distance L varies, the drop .DELTA.P in the pressure in the secondary port 14 which is required for the seat 38 to be spaced from the seal pad 32 also varies, resulting in a change in the rate of the fluid flowing in the pressure-reducing valve 10 varies. When a plurality of pressure-reducing valves 10 are manufactured, it is difficult to keep them at a constant quality level. Accordingly, it is also difficult to keep fluid pressure systems which incorporate such pressure-reducing valves 10 at a constant quality level.